1. Field of the Invention
The present invention relates to a wiring board for mounting a semiconductor element or the like such as a semiconductor integrated circuit device.
2. Description of Related Art
A built-up wiring board is used as a wiring board used for mounting a semiconductor element such as a semiconductor integrated circuit device. For example, a plurality of insulating layers and a plurality of copper-plated layers are laminated on a core layer, thereby forming the built-up wiring board.
At an upper surface of the wiring board, semiconductor element connection pads connected to electrodes of the semiconductor element are arranged in a lattice pattern. In contrast, at a lower surface of the wiring board, external connection pads connected to an external electric circuit board are arranged in a lattice pattern. The semiconductor element connection pads and the external connection pads are electrically connected to each other via wiring conductors. A solder bump for joining the electrode of the semiconductor element to the semiconductor element connection pads is welded onto the semiconductor element connection pads.
In the above-described wiring board, the semiconductor element is mounted at the upper surface of the wiring board such that each of the electrodes for the semiconductor element abuts against the corresponding solder bump, and further, these members are heated such that the solder bump is melted, so that the solder bump is joined to the electrode of the semiconductor element, thus mounting the semiconductor element on the wiring board.
In the above-described wiring board, the characteristic impedance of a signal wiring conductor for propagating a signal is set to about 50Ω in the case of a single end signal whereas about 100Ω in the case of a difference signal in order to suppress damping of a signal to the minimum. In contrast, the input impedance of the semiconductor element is several hundred megohms or more whereas the output impedance thereof ranges from several ohms to several tens of ohms, and therefore, the characteristic impedance of the semiconductor element is markedly different from the characteristic impedance of the wiring conductor. For this reason, in the case where a high-speed signal is transmitted to or received from the semiconductor element through the signal wiring conductor, noise having a reflection wave overlapping an input/output terminal occurs at the signal transmitted through the signal wiring conductor. As a result, there has arisen a problem that the noise inhibits the semiconductor element from being normally operated.
In view of this, in order to solve the above-described problem, as shown in FIGS. 7 and 8, there has been proposed a wiring board including a semiconductor element connection pad 13, in which the following layers are laminated in sequence on an insulating layer 11b: a main conductor layer 13a containing a low resistance material having a volume resistivity of 100 μΩ·cm or less; a resistor layer 13b containing a high resistance material having a volume resistivity of 10 Ω·cm or more; and a coating layer 13c having a high soldering wettability, wherein the main conductor layer 13a, the resistor layer 13b, and the coating layer 13c are electrically connected in series to each other. FIG. 8 shows a wiring board, in which a solder resist layer 19 is formed on the insulating layer 11b shown in FIG. 7 (e.g., Japanese Unexamined Patent Application No. 2013-45938).
The resistor layer 13b electrically connected in series to the main conductor layer 13a forms a damping resistance, which can damp noise caused by a difference in characteristic impedance between the wiring conductor and the semiconductor element so as to normally actuate the semiconductor element. The semiconductor element connection pad 13 is connected to the wiring conductor at the lower layer via a via conductor 12b containing a low resistance material integrally connected to the lower surface of the main conductor layer 13a and having a volume resistivity of 100 μΩ·cm or less.
However, the resistor layer 13b formed between the main conductor layer 13a and the coating layer 13c is as thin as 100 nm to 1000 nm, and therefore, the melted solder bump strides over the resistor layer 13b between the main conductor layer 13a and the coating layer 13c at the side surface of the connection pad, thereby possibly inducing short-circuiting. In this manner, if short-circuiting occurs, the function of the damping resistance cannot effectively act, resulting in the fear that noise caused by the difference in characteristic impedance between the wiring conductor and the semiconductor element cannot be damped, so that the semiconductor element cannot be normally actuated.